1. Field of the Invention
Embodiments of the present invention generally relate to a method and apparatus of acoustically transmitting data to and from downhole environments.
2. Description of the Related Art
To recover oil and gas from subsurface formations, wellbores/boreholes are drilled by rotating a drill bit attached at an end of a drill string. The drill string includes a drill pipe or a coiled tubing (referred herein as the “tubing”) coupled to a bottomhole assembly (BHA) which, in turn, carries the drill bit at its end. The drill bit is rotated by, for example, operation of a mud motor disposed in the BHA. In this case, a drilling fluid commonly referred to as the “mud” is supplied under pressure from a surface source into the tubing during drilling of the wellbore and through the mud motor. The pressurized drilling fluid (mud) acts as a motive fluid to operate the mud motor and is then discharged at the drill bit bottom. The drilling fluid then returns to the surface via the annular space (annulus) between the drill string and the wellbore wall or casing wall. In addition to operating the mud motor, the drilling fluid serves to clean the workface at the bit and carry the drill cuttings back to the surface, lubricate and cool the drill bit, and stabilize the wellbore that is formed to prevent its collapse.
From time to time, conditions may arise which mitigate the effectiveness of the motor of a drill string in performing its above listed functions and may even damage the motor. For example, the motor may stall during operation. A motor may stall for a number of reasons including setting down too much weight-on-bit, running into a tight area and pinching the bit-box, a stator failure, etc. It is both expensive and time-consuming to pull the motor out of the wellbore each time there is doubt as to whether the motor is turning.
Another undesirable condition which may arise downhole is a leak between the interior and the exterior of the drill pipe to create a “short circuit” which reduces the effectiveness of the drilling fluid in performing its functions. If such a leak goes undetected and is allowed to persist over time, the flow of the drilling fluid, which is typically loaded with solids, will erode or wash away enough of the material of the drill pipe at the location of the leak as to weaken the pipe to the point of separation (twist off). Lost pipe in the bottom of the well prevents further drilling of the well until such time as the separated portion is retrieved or “fished” from the well. Fishing operations are time consuming and expensive and not always successful. If unsuccessful, the well must be abandoned and a new well or a sidetrack begun. Even if successful, the fishing operation presents a significant financial loss.
Another detrimental event that may occur is a flow restriction or blockage, which also interferes with the effectiveness of the drilling fluid. Furthermore, a total blockage has been known to cause a rapid increase in hydraulic pressure in the drill string with eventual rupture of the drill string or the standpipe which feeds the drilling fluid to the drill string at the earth's surface. Again, such a condition inhibits successful drilling and results in increased operating expenses.
As a result of these and other conditions which may occur downhole, there is a need for effectively monitoring and characterizing the motor system of a drill pipe. Conventionally, the relevant operating parameters which are observed during operation of a motor during drilling include torque, RPMs, pressure and flow. These parameters may be used individually or collectively to characterize the operation of the motor. For example, in the event of a motor stall, blockage or restriction the pressure drop in the motor is expected to increase above the operating pressure. As another example, RPMs and torque of a positive displacement motor are computed using information on flow rate and pressure drop. Such a computation is facilitated by characteristic curves contained in performance charts provided by manufacturers of downhole motors. However, such approaches are not always accurate. For example, depending on the particular problem, the pressure may not exhibit any change, regardless of the condition of the motor. Furthermore, there is a significant time delay in the pressure indication when drilling with a compressible medium, such as in the case of underbalanced drilling using nitrogen.
Another technique for monitoring and characterizing the operation of a motor downhole is by acoustics. For example, one approach is to determine drill bit speed by isolating the rotor whirl frequency of a progressive cavity motor. However, this technique is limited because some motors do not create a strong acoustical signature all the time. Often, it is not possible to acoustically differentiate a stalled motor from a rotating motor.
Therefore, there is a need for a method and apparatus for monitoring and characterizing the operation of a motor downhole. Preferably, the monitoring and characterization occurs in real-time so that continues efficient motor operation can be insured.